This invention generally relates to display media, and the preparation of display media, and to devices using such display media. In particular, this invention relates to display media and displays for which the image remains in view after the field and/or power used to form the image is eliminated (completely reduced to zero), or reduced (decreased to a level below normally required to form the image). The image is formed by switching materials in the pixels between two states (such as, for example, black and white). In embodiments, the display media comprise bichromal beads. In embodiments, the bichromal beads comprise crystalline materials contained therein. The display media made with bichromal beads, for example Gyricon beads, are useful in generating images which can be stored or erased, and function by rotating a bichromal sphere by an external field to create the image.
Display media, such as Electric Paper or twisted ball panel display devices, are known and are described, for example, in U.S. Pat. Nos. 4,126,854; 4,143,103; 4,261,653; 4,438,160; 5,389,945. The media generally are comprised of an encapsulant material, for example, an elastomer, such as a cured polysiloxane, sandwiched between two indium tin oxide coated substrates, such as glass or MYLAR(trademark). Generally, the elastomer layer has closely packed cavities, each containing a bichromal sphere suspended in a dielectric liquid. The dielectric liquid may also be present in substantial amounts in the elastomer matrix. In media that are active in an electric field, the bichromal spheres have a net dipole due to different levels of charge on the two sides of the sphere. An image is formed by the application of an electric field to each pixel of the display, which rotates the bichromal spheres to expose one color or the other to the viewing surface of the media. The spheres may also have a net charge, in which case they will translate in the electric field as well as rotate. When the electric field is reduced or eliminated, the spheres ideally do not rotate further; hence, both colors of the image remain intact. This image bistability is one feature of display media made with bichromal Gyricon beads.
The fabrication of certain bichromal spheres is known, for example, as set forth in the above mentioned U.S. Pat. No. 4,143,103 patent, wherein the sphere is comprised of black polyethylene with a light reflective material, for example, indium, sputtered on one hemisphere. Also in U.S. Pat. No. 4,438,160, a rotary ball is prepared by coating white glass balls of about 50 microns in diameter, with an inorganic coloring layer such as co-deposited MgF2 and chromium by evaporation. In a similar process, there is disclosed in an article entitled xe2x80x9cThe Gyriconxe2x80x94A twisting Ball Displayxe2x80x9d, published in the proceedings of the S.I.D., Vol. 18/3 and 4 (1977), a method for fabricating bichromal balls by first heavily loading chromatic glass balls with a white pigment such as titanium oxide, followed by coating from one direction in a vacuum evaporation chamber with a dense layer of nonconductive black material which coats only one hemisphere.
Also in U.S. Pat. No. 4,810,431 by Leidner, there is disclosed a process for generating spherical particles by (a) coextruding a fiber of a semi-circular layer of a polyethylene pigmented white and a black layer of polyethylene containing magnetite, (b) chopping the resultant fiber into fine particles ranging from 10 microns to about 10 millimeters, (c) mixing the particles with clay or anti-agglomeration materials, and (d) heating the mixture with a liquid at about 120xc2x0 C. to spherodize the particles, followed by cooling to allow for solidification.
In another method, the bichromal beads used in the fabrication of display media such as Gyricon electric paper are formed by wetting the top and bottom surfaces of a spinning disk with two different pigmented molten solids. These streams combine at the edge of the disk and, driven by a Taylor instability, they form a series of jets emanating from the edge of the disk. In particular, a 3 inch diameter disk will have about 300 such jets. Each jet is seen with high speed video to be comprised of two very distinct parts corresponding to the two pigmented liquids used, with no apparent mixing within the jet. The jets subsequently break up into spheres by the Rayleigh instability. Again, with high speed video, is can be seen that close to the jet break-up points, these spheres are very high quality, hemispherical bichromal spheres.
When the beads are later collected, having hardened in air before reaching the base plate, they are found to have a degree and a percentage of imperfections that were not seen when they were first formed. Some of these imperfections were obviously caused by collisions of still-molten beads. Specifically, larger beads move longer distances before slowing as compared to smaller beads, thereby causing distributions of velocities that lead to collisions. In addition to collisions, other imperfections such as engulfing may result. Engulfing is when one liquid will flow partly or completely over the other liquid. This is caused partly by surface tension differences and partly by viscosity differences. Such engulfing is not the result of collisions, but rather, is the result of interactive flow of the two liquids after initial bead formation. Both of these conditions, collisions and engulfing, are greatly minimized by causing the beads to solidify quickly after initial formation.
Therefore, it is desired to provide a process for making bichromal beads which does not result in the imperfections discussed above.
The present invention provides, in embodiments, for bichromal sphere formulations that include crystalline materials in order to help reduce the imperfections caused during the process of making bichromal beads.
Embodiments of the present invention include: a display media comprising a) an encapsulant medium, and b) bichromal beads comprising a crystalline material, wherein the bichromal beads are dispersed in the encapsulant medium.
Embodiments also include: a display media comprising a) an encapsulant medium, and b) bichromal beads comprising a crystalline polyethylene wax having a molecular weight of from about 400 to about 4,000, and wherein the bichromal beads are dispersed in the encapsulant medium.
Embodiments further include: a display apparatus capable of causing an image to be displayed on a display media comprising: a) an encapsulant medium, and b) bichromal beads comprising a crystalline material, wherein said bichromal beads are dispersed in said encapsulant medium; and a means for orienting said bichromal beads in said substrate so as to form an image on said display media.
The present invention relates to twisting ball display media containing bichromal beads or balls. The display media comprises a media that, in embodiments, has many attributes of a paper document. For example, in embodiments, the media may look like paper, have ambient light behavior like paper, be flexible like paper, be carried around like paper, be copied like paper, and have nearly the archival memory of paper.